Prostate cancer is a major health problem. Unfortunately, we do not as yet have effective methods for prostate cancer prevention. The best strategy that we have now for prostate cancer is early diagnosis followed by definitive treatment. This method significantly reduces prostate cancer mortality. The most widely used strategy for early diagnosis of prostate cancer includes measurement of prostate-specific antigen in serum. This method identifies men at high risk who, subsequently, are biopsied to confirm the diagnosis. Unfortunately, the test is not very powerful and approximately 5 patients have to be biopsied to detect 1 prostate cancer. Clearly, there is a need for development of more biomarkers for diagnosis of prostate cancer. We have recently discovered a new gene that belongs to the same family as PSA and has other characteristics which are also similar to PSA, i.e. the gene encodes for a secreted protease, the gene is regulated by androgens, is highly expressed in the prostate and the protein is secreted into the circulation. We speculate that the levels of the encoded protein, human kallikrein 4, may also be elevated in prostate cancer and that the measurement of hK4, in addition to PSA and other biomarkers, may significantly improve our ability to specifically diagnose early prostate cancer. In this grant application, we propose to develop the necessary technology for measuring human kallikrein 4 in serum and perform exploratory clinical studies to investigate if, indeed, the concentration of this biomarker is increased in prostate cancer and if its combination with other biomarkers can improve the diagnostic accuracy. If successful, our research will contribute to the development of new prostatic biomarkers and possibly, to better diagnosis which may lead to better clinical outcomes and reduced healthcare costs.